The Observed Time Difference of Arrival (OTDOA) feature was introduced in the LTE R9 standard as a user equipment (UE)-assisted positioning method. It comprises estimating the location of a UE by using the positioning reference signal arrival times that the UE measures from a number of surrounding eNodeBs.
A UE is provided with a list of cells by the location server of a cellular network to measure the relative arrival time with respect to a pre-configured cell (most often the serving cell). The relative arrival time is referred to as reference signal time difference (RSTD), and the pre-configured cell is referred to as the reference cell. Each cell in the list is associated with a priori information on its RSTD. This information is composed of an expected RSTD, which is estimated by the network, and an uncertainty window centered around the expected RSTD value which helps the UE to localize its search in the uncertainty window only.
The network, or precisely the location server, will use the set of RSTDs measured by the UE to obtain the final estimate of the UE position. Therefore, the quality of this positioning method depends closely on the accuracy of the RSTD measurement performed by the UE.
The UE measures RSTD using the Positioning Reference Signal (PRS) that is transmitted on antenna port (AP) 6 of a measured cell. As per standard specification, the size of the uncertainty window can be 2×3069 Ts and the expected RSTD can be 3×8192 Ts (approximately 11 OFDM symbols). Therefore, it is common for the PRS subframe of a measured cell not to be aligned in time with that of the reference cell. Common reasons for such a misalignment include high distance between the serving and measured eNodeB, or asynchronous operation.
Since frequency-domain subframe-based processing is the reference baseband processing used in LTE, a frequency-domain implementation should be less complex than a time-domain approach for RSTD estimation by the UE.
Typically, the underlying baseband processing is synchronized with the serving cell. Keeping the same data path, the frequency-domain data is only available per useful part of an OFDM symbol (i.e. without the cyclic prefix) aligned in time with the serving cell. Thus, a frequency-domain implementation of OTDOA measurement typically includes the following parts:
1. Determine the reference OFDM symbol that corresponds best to the positioning symbol of a measured cell by using an estimation technique,
2. Descramble the selected reference OFDM symbol with the scrambling sequence that is used for PRS of the measured cell,
3. Estimate the PRS arrival time of the measured cell, for example by using an IFFT transform of the resulting descrambled frequency-domain data.
A problem of frequency-domain implementation is that each OFDM symbol can only cover a maximum timing range of 2048 Ts. This is due to the fact that in the frequency domain the timing measure is circular so that we can only get a time measure modulo 2048 Ts which is the FFT size (to enable frequency-domain processing as would be understood).
Taking the following example to illustrate the limiting range, in FIG. 1, a measured cell's positioning symbol is assumed to be in different locations with respect to the reference OFDM symbol n: 1024 Ts earlier in example (a); perfectly aligned in example (b); 1023 Ts later in example (c); and (1023+τ)Ts later in example (d).
For ease of explanation, it is assumed that the channel is composed of only one tap. Then, the power-delay profile (PDP) obtained from the reference OFDM symbol n is as in FIG. 2 where the channel tap position of each corresponding case in FIG. 1 is given.
Since the measured delay is cyclic and only measured modulo 2048 Ts, and with the range covered by a window, for example [0, 2048 Ts], or [−1024 Ts, 1023 Ts], a delay not falling in this range will be estimated wrongly, i.e. with an error that is a multiple of 2048 Ts. In the example of FIG. 2, the covered range is [−1024 Ts, 1023 Ts]. To measure a delay that is greater than 1023 Ts, (position (d)), there will be a systematic error.
Moreover, by convention, if only timings in [−1024 Ts, 1023 Ts] are measured, the FFT window is represented by inverting its left and right halves so that the 0 Ts delay appears to be in the center of the FFT window. This representation is used in FIG. 2. With this convention, any delay in [1024, 2048] is located in the left half of the window, see (d) in FIG. 2. The positions of (a), (b), (c), and (d) inside FIG. 2 are given using these conventions.
For instance, the positioning symbol at (b) results in a channel tap at delay 0 Ts. At position (d), the channel tap appears in the negative part of the PDP at position (τ−1)Ts from −1024 Ts due to phase wrapping. This situation shows the fact that it is ambiguous whether channel tap in the resulting PDP corresponds to a real negative delay or a wrapping positive delay. Similarly, there is the same issue the other way round, where a signal having a real negative delay of (−1024−τ)Ts will appear as having a wrapped delay of (1023−τ)Ts 
In relation to position (d) in FIG. 1, for τ>CP/2, reference OFDM symbol n+1 (10) contains the positioning symbol better than reference OFDM symbol n (11) does. In this case, symbol n+1 is preferably selected as the best symbol for the positioning symbol. However, estimation imperfection as well as noise effect may result in reference symbol n but not reference symbol n+1 as the selected symbol. This effect of imperfect symbol selection adds another source of ambiguity in the RSTD estimate.
The above examples can be generalized to show that a frequency-domain implementation faces ambiguity regions as illustrated in FIG. 3. Essentially, ambiguity systematically occurs in the regions (30, 31) where the real delay exceeds the range of each PDP.
The aforementioned ambiguity regions are inherent to the limiting timing range of the PDP obtained from one reference OFDM symbol. Could this ambiguity problem be avoided by using a PDP that is computed from a combination of several reference OFDM symbols instead of from only one?
Let us briefly describe some (but not all) types of combinations to see what combination means. Take example (c) of FIG. 1, the data of the positioning symbol is partially captured in reference OFDM symbols n and n+1. A possible way to proceed is to firstly construct the data of the positioning symbol by coherently combining the frequency-domain data of reference symbols n and n+1 taking into account phase different between the two symbols, and then compute the PDP from this resulting combined frequency-domain data. Another possibility is to combine the two data sets output from IFFT of each reference symbol n and symbol n+1, and then combine these two data sets either by complex value combination or by power combination in order to get a combined PDP at the end.
For such a combined PDP, it is beneficial but not essential to define its timing range as [0,2047], as shown in FIG. 4, to express the fact that the positioning symbol is supposedly laying in between reference symbols n and n+1.
In light of the above, ambiguity still occurs even with a combined PDP. Let us consider combined PDP obtained from reference OFDM symbols n and n+1 as in FIG. 4. Here, position (b) of the positioning symbol in FIG. 1 results in channel tap at position 0 Ts, and position (d) results in channel tap at position (1023+τ)Ts. Similarly, we have channel tap at position 1023 Ts for position (c). On the contrary, position (a) of the positioning symbol results in wrapping channel tap at position 2047−1024=1023 Ts, i.e. it is identical to position (c). This shows that ambiguity of the delay estimate can still occur.
It can be generalized to show that the PDP obtained from the symbol combined from reference symbols n and n+1 has ambiguity regions as shown in FIG. 5.
As a consequence, ambiguity is an issue of the frequency-domain approach.
In a typical frequency-domain implementation of OTDOA receiver, PRS pilot tones corresponding to each eNodeB are extracted in the frequency domain, descrambled and transformed back into time domain on an OFDM symbol basis. Then, accumulation or interpolation of the time domain symbols is performed for all PRS-carrying symbols of a particular subframe to obtain one power delay profile on which timing estimation will be done. The operations of interpolation/accumulation and iFFT may be inverted i.e. carried out in a different order. Then a UE search window is placed around the highest correlation value for the search of the first channel path.
However, all frequency-domain implementations proposed so far have only considered small uncertainty range for which the ambiguity issue is not addressed, whereas time-domain implementations allow performance gain but are known to be computationally more costly.
Accordingly, there is a need to provide a method for ambiguity resolution of RSTD estimation in a frequency-domain implementation of OTDOA measurements in an LTE network.